FIG. 1 is a network architecture of a long term evolution (LTE) system which is the related art mobile communication system has evolved from the existent UMTS system and a basic standardization therefor is undergoing in 3GPP.
The LTE network may be divided into evolved UMTS terrestrial radio access network (E-UTRAN) and core network (CN). The E-UTRAN includes a terminal (User Equipment; UE), a base station (Evolved Node B; eNB), an access gateway (aGW) located at the end of the network to be connected to an external network. The aGW may be divided into a portion of handling a user traffic and a portion of processing a control traffic. Here, a new interface may be used for the communication between the aGW for processing the user traffic and the aGW for processing the control traffic. One or more cells may exist in one eNB. An interface for transmission of the user traffic or control traffic may be used between eNBs. The CN may include an aGW, a node for a user registration of other UEs and the like. An interface may be used to identify the E-UTRAN and CN.
FIG. 2 is an architecture of a radio interface protocol control plane between a terminal and an E-UTRAN based upon the 3GPP radio access network standard, and FIG. 3 is an architecture of a radio interface protocol user plane between a terminal and an E-UTRAN based upon the 3GPP radio access network standard.
Hereinafter, the architecture of radio interface protocols between the terminal and the E-UTRAN will be described with reference to FIGS. 2 and 3.
The radio interface protocol has horizontal layers comprising a physical layer, a data link layer and a network layer, and has vertical planes comprising a user plane for transmitting data information and a control plane for transmitting a control signaling. The protocol layers can be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based on three lower layers of an Open System Interconnection (OSI) standard model widely known in communications systems. Such radio interface protocols may exist as a pair between the terminal and the E-UTRAN, to manage data transmissions over interfaces.
Hereinafter, each layer in the radio protocol control plane in FIG. 2 and the radio protocol user plane in FIG. 3 will be described.
A first layer, as a physical (PHY) layer, provides an information transfer service to an upper layer using a physical channel. The physical layer is connected to its upper layer, called a Medium Access Control (MAC) layer, via a transport channel. The MAC layer and the physical layer exchange data via the transport channel. Here, the transport channels may be divided into a dedicated transport channel and a common transport channel depending on whether the transport channel is shared. Data is transferred via a physical channel between different physical layers, namely, between the physical layer of a transmitting side and the physical layer of a receiving side.
Various layers exist in the second layer. First, a medium access control (MAC) layer serves to map different logical channels to different transport channels, and also performs a logical channel multiplexing for mapping several logical channels to one transport channel. The MAC layer is connected to an upper radio link control (RLC) layer via a logical channel. Logical channels are divided according to a type of information to be transmitted into a control channel for transmitting control plane information and a traffic channel for transmitting user plane information.
The RLC layer of the second layer manages segmentation and concatenation of data received from an upper layer to appropriately adjust a data size such that a lower layer can send data over an interface. Also, the RLC layer provides three operation modes, including a transparent mode (TM), an un-acknowledged mode (UM) and an acknowledged mode (AM), so as to guarantee various quality of service (QoS) requirements of each radio bearer (RB). In particular, the RLC layer operating in the AM mode (hereinafter, referred to as AM RLC layer) performs a retransmission using an automatic repeat and request (ARQ) function for a reliable data transmission.
A packet data convergence protocol (PDCP) layer located at the second layer is used to efficiently transmit IP packets, such as IPv4 or IPv6, on a radio interface with a relatively narrow bandwidth. For this purpose, the PDCP layer reduces the size of an IP packet header which is relatively great in size and includes unnecessary control information, namely, performs a function called header compression. Accordingly, only necessary information can be included in the header part of data for transmission, so as to increase a transmission efficiency of a radio interface.
A radio resource control (RRC) layer located at the lowermost portion of the third layer is only defined in the control plane. The RRC layer controls logical channels, transport channels and physical channels in relation to configuration, re-configuration and release of Radio Bearers (RBs). Here, the RB denotes a logical path that the L2 layer provides for data transmission between the terminal and the UTRAN. In general, the establishment of the RB refers to stipulating the characteristics of protocol layer and channel required for providing a specific service, and setting the respective detailed parameters and operation methods. The RBs are divided into a signaling RB (SRB) and a data RB (DRB). The SRB is used as a path for transmission of RRC messages in the C-plane, while the DRB is used as a path for transmissions of user data in the U-plane.
Hereinafter, the RLC layer will be described in more detail. The RLC layer provides three modes, such as the TM, UM and AM, as mentioned above. The RLC layer rarely performs a function in the TM, and thus UM and AM will only be described herein. The UM RLC adds a protocol data unit (PDU) header including a sequence number (SN) to each PDU for transmission, such that a receiving side can be known as to which PDU has been lost during transmission. Due to such function, the UM RLC manages, in the user plane, the transmission of multimedia data or the transmission of real-time packet data, such as voice (e.g., VoIP) or streaming in a packet service domain (hereinafter, referred to as a PS domain), while managing, in the control plane, the transmission of an RRC message, which does not need a reception acknowledgement, among RRC messages sent to a specific terminal or specific terminal group within a cell.
Similarly, the AM RLC constructs a PDU by adding a PDU header including an SN upon the construction of PDU. Unlike the UM RLC, a receiving side acknowledges a PDU sent by a transmitting side. The receiving side acknowledges in order to request a retransmission of unsuccessfully received PDU from the transmitting side. Such retransmission function is the most important characteristic of the AM RLC. Thus, the AM RLC aims to guarantee an error-free data transmission via the retransmission. Under the purpose, the AM RLC usually manages a non-real-time packet data transmission, such as TCP/IP of PS domain, in the user plane, while managing a transmission of RRC message, which requires a reception acknowledgement, among RRC messages transmitted to a specific terminal within a cell in the control plane.
From the perspective of direction, the UM RLC is used for a uni-directional communication, while the AM RLC is used for a bi-directional communication due to a feedback from a receiving side. From the structural perspective, there is a difference, namely, the UM RLC is configured such that one RLC entity performs transmission or reception while the AM RLC is configured such that both transmitting side and receiving side exist in one RLC entity. The complicated configuration of the AM RLC is due to the retransmission. The AM RLC includes a retransmission buffer for managing the retransmission, in addition to a transmission/reception buffer. Also, the AM RLC performs various functions, such as using transmitting and receiving windows for a flow control, polling for a transmitting side to request status information from a receiving side of an RLC entity, sending a status report for a receiving side to report its buffer state to a transmitting side of a peer RLC entity, constructing a status PDU for delivering status information, and the like. The AM RLC also needs various protocol parameters, such as status variables and a timer, in order to support the functions. A PDU, such as status report or status PDU, which is used for controlling the data transmission in the AM RLC, is referred to as ‘Control PDU’, and a PDU used for transferring user data is referred to as ‘Data PDU’.
An RLC data PDU in the AM RLC may be divided into AMD PDU and AMD PDU segment, in detail. The AMD PDU segment has part of data included in the AMD PDU. In the LTE system, a maximum size of a data block is changeable every time a terminal sends the data block. Hence, after a transmitting side AM RLC entity constructs a 200-byte AMD PDU at a specific time and transmits the constructed AMD PDU, when the transmitting side AM RLC receives NACK from a receiving side AM RLC and thereby tries to retransmit the AMD PDU, if a maximum size of data block to be actually transmittable is 100 bytes, the same AMD PDU cannot be sent as it is. In this case, the AMD PDU segment is used. The AMD PDU segment denotes that the corresponding AMD PDU is segmented into smaller units. During the procedure, the transmitting side AM RLC entity divides the AMD PDU into the AMD PDU segments and transmits the AMD PDU segments over several transmission time intervals. The receiving side AM RLC entity then restores the AMD PDU from the received AMD PDU segments.
If there is unsuccessfully (incompletely or incorrectly) received data, the receiving side AM RLC requests a retransmission of such data from the transmitting side AM RLC, which is referred to as ‘status report’. The status report is sent by using STATUS PDU, which is one of control PDUs.